Solid-state power converters often are used to provide high voltage current from generators or alternators to electrical loads such as vehicle motors. Such power converters include semiconductor power switches to commutate current and may also include capacitive and/or inductive elements to assist in adjusting voltage. It will be appreciated that power converters also can be used in many other settings.
Generally, power converters are operated by applying alternately two different gate voltage levels to individual semiconductor power switches via corresponding gate drive units. One (higher) gate voltage forward-biases or drives a switch, whereas the other (lower or reversed) gate voltage turns off the switch. Semiconductor power switches are limited, however, in how much current they can conduct. While forward-biased, each power switch conducts significant current in a forward direction at a relatively small voltage drop across the switch. Despite the relatively low voltage across the forward-biased power switch, resistive heating can occur. As a result, for large electrical loads, it can be necessary to operate a power converter with plural switches connected in parallel. Often such a parallel power converter will be arranged in modules, each module being an inverter in itself. The modular inverters then are coordinated by a control system.
Although durability is a consideration in semiconductor switch design, electrical design constraints entail that the various layers of the semiconductor power switches are fabricated from materials having differing thermal properties, in particular, differing coefficients of thermal expansion. Therefore, over time, thermal stress can potentially cause delamination, debonding of terminals, or fatigue cracking. Thermal stress can also cause electrochemical failures such as current filamenting and Kirkendall void formation.
Thermal stress effects can be rendered more predictable, and can be mitigated, by maintaining the heating/cooling cycle within a design envelope defined to minimize temperature swings despite continual on/off cycling.
Often, switch electrical connection points are split into multiple terminals to allow for high currents. An example of this is when a power converter for large current loading is constructed of multiple inverters connected in parallel (a multi-inverter power converter or “multi-converter”). Multi-converters present a known chaotic system, in which small deviations from synchronous switching of the parallel inverters can lead to large fluctuations in electrical performance and current flow. Indeed, synchronization presents a key challenge for using devices in parallel—even a few microseconds of timing offset can create significant excess current flow in parts of a circuit and can drive component failures.
In view of the above, it may be desirable to improve synchrony of switching among the multiple inverters in a multi-converter, in order to mitigate unexpected circuit oscillations.